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GEOMgTq(x Ms. Katie Hower Chevron Pipe Line Company 19 March 1997 Page 2 distribution of the residual petroleum is above and below the current water table, probably due to historical fluctuations in the water table. This distribution of petroleum below the water table allows for a direct assessment of its potential threat to groundwater, because the presence of petroleum in contact with groundwater functionally represents a long-term in situ leaching test. Old Valley Pipeline sites are assumed to have reached chemical equilibrium or steady-state conditions because of the age of the release. INVESTIGATION/MONITORING APPROACH At each site impacted by residual petroleum from the Old Valley Pipeline, CPL will evaluate the potential threat to groundwater and, if applicable,risks to human health. Because the potential source is essentially the same between sites, a consistent approach will provide data for this evaluation. Based on the lessons learned from investigations at several Old Valley Pipeline sites, the following approach will be used, as discussed below and presented in a flow chart format (Figure 2); analytical requirements are discussed in the next section. The data requirements for each of these steps may be met by using historical site data, a new data collection program, or by a combination thereof. 1. The lateral and vertical extent of affected soil shall be defined and preliminary (or screening level) groundwater information shall be obtained using either historical or newly acquired data. A soil boring/grab groundwater sampling method is acceptable to provide this data. The petroleum-affected area must be clearly constrained. Several groundwater samples will be collected immediately downgradient of affected soil (if gradient direction is known). If gradient direction is unknown, groundwater samples will be collected from locations such that a plume would be detected (if one exists). A groundwater sample will be collected from within the affected soil area. The potential for interferences to the groundwater measurements by petroleum-affected particulates and/or biogenic byproducts will be addressed using filtering and/or silica gel cleanup steps(see following discussion). 2. If all groundwater sample results are non-detect or less than RWQCB Water Quality Objectives (WQOs), then the site will be identified as a"soil only" (vadose zone) case and the County Health Department(or equivalent) will provide regulatory oversight. Potential risks to human health posed by residual petroleum in soil will be evaluated under appropriate land use scenarios. 3. If groundwater sample results meet or exceed WQOs, the site will be identified as a "groundwater" case and the lateral and vertical extent of dissolved petroleum in groundwater will be determined using a screening method. 4. Monitoring wells will be installed as per the distribution of dissolved petroleum identified by the screening data and a quarterly monitoring program will be implemented.